Microwave filters are often in form of so-called cavity resonators where the resonances in a hollow cabinet are utilized for filtering specific frequencies. The cavity of the cabinet can be shaped in many ways. An input coaxial conductor conducts alternating currents with differing frequencies to a cabinet. Columnar resonators are provided in the cabinet, said resonators being of a length typically corresponding to one fourth of the wavelength of the frequency at which the filter is to operate.
The inner conductor of the input coaxial connector is connected to an input resonator, and the inner conductor of the output coaxial connector is connected to an output resonator. A plurality of columnar resonators is arranged in the filter cavity between the input resonator and the output resonator, and at a first end these columnar resonators are secured to the bottom of the cabinet. Together with a so-called trimming screw, each columnar resonator defines a capacitor at its free ends. A turning of the trimming screw renders it possible to adjust the capacitance and consequently the properties of the filter. When all the resonators are secured to the same surface, such as the bottom of the cabinet, the filters are called combline filters, and when all the resonators are alternately secured to two opposing surfaces, said filters are called interdigital filters. The outer conductor of both the input coaxial connector and the output coaxial connector is connected to the metal cabinet.
A filter of the above type can be in form of a passive filter without an electric amplification, or it can be in form of an active filter where one of the resonators is connected to the input of an electric amplification circuit amplifying the signal, and where the output of the amplification circuit is connected to a subsequent resonator.
The patent literature reveals many examples of such microwave filters. U.S. Pat. No. 4,091,344 (Wavecom Industries) discloses a microwave filter of the combline type, and U.S. Pat. No. 3,818,389 (Bell Telephone Laboratories) discloses a microwave filter of the interdigital type.
Modern microwave filters are often manufactured with a cabinet and resonators of solid metal, and they are optionally formed integral by being milled out of a metal block. The microwave filter can be mounted outdoors on a building or an aerial mast for instance for mobile telephony, and accordingly said microwave filter is subjected to wind and weather and other environmental strains. Accordingly, it is of vital importance that the structure is strong. Strokes of lightning are particularly problematic because it must be possible to divert or conduct very strong lightning currents away from vulnerable components. The microwave filters on aerial masts are often mounted close to the aerial, a coaxial cable connecting the aerial to the coaxial input of the microwave filter. The filter only allows frequencies within a specific frequency band to pass, optionally in an amplified form, to the output coaxial connector while the remaining frequencies are filtered off. In connection with strokes of lightning, the lightning current runs preferably on the outer side of the outer conductor of the coaxial cables. As the outer conductors of the coaxial connectors are directly connected to the metal cabinet of the filter, the lightning current can easily run from the outer conductor of the input coaxial connector, through the outer conductor of the output coaxial connector and further to the ground or vice versa without causing major damage to the components of the microwave filter. The metal cabinet presents often a material thickness of 1 to 10 mm.
In order to reduce the price and the weight of the microwave filters of this type, said filters can be made of plastics, such as by way of moulding, where the plastic surface is subsequently coated with a metal capable of conducting the electric signals to be subjected to a filtering. The coating process can be in form of activation by means of palladium, autocatalytic deposition of nickel or copper followed by an electrolytic deposition of for instance silver. Such a metal coating is in practice often of a thickness of 5 to 200 μm, which suffices completely for conducting aerial signals of an average effect of up to a few hundred watts. However, a strong lightning current can cause a damaging heating of the metal coating due to the relatively highohmic resistance and low heat capacity of said metal coating.
U.S. Pat. No. 5,502,715 (Penny) discloses a microwave filter which can be made of metallised plastics. According to this publication the risk of potential damages caused by strokes of lightning can be reduced by grounding the ports of the filter. The microwave filter of this publication is in form of a diplex filter including a first coaxial connector and a second coaxial connector, where both coaxial connectors are used as input and output connectors, respectively.
U.S. Pat. No. 5,508,873 discloses a surge protector for broadband coaxial systems, wherein the cabinet is made of metal or metallised plastics. It is not mentioned in this patent how the metallisation layer can be protected from a strong lightening current running between the outer conductors of the coaxial connectors and causing a damaging heating of the metallisation layer.